DHC Working Group Bernard Aboba
INTERNET-DRAFT Microsoft Corporation
Category: Best Current Practice
<draft-ietf-dhc-dna-ipv4-01.txt>
11 September 2003Detection of Network Attachment (DNA) in IPv4
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This specification attempts to synthesize experience garnered over the
years in the deployment of hosts supporting ARP, DHCP and IPv4 Link-
Local addresses. Given this experience, this document suggests
optimizations for detection of network attachment in IPv4 as well as
heuristics for determining when assignment of an IPv4 Link-Local address
is appropriate.
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INTERNET-DRAFT DNAv4 11 September 20031. Introduction
This draft attempts to synthesize experience garnered over the years in
the deployment of hosts supporting ARP [RFC826], DHCP [RFC2131], and
Link-Local IPv4 addresses [IPv4LL]. Experience has indicated the
importance of several goals in detection of network attachment:
[a] Avoiding inappropriate assignment of Link-Local IPv4 addresses.
Experience has shown that in the vast majority of cases, the
assignment of Link-Local IPv4 addresses is inappropriate. That
is, the IPv4 host assigning an Link-Local IPv4 address either
is not connected to a network at all, in which case assignment
of an Link-Local IPv4 address does no good; or the host is in
fact present on a network with a DHCPv4 server but for one
reason or another does not receive a response to a DHCPREQUEST
or DHCPDISCOVER.
[b] Latency Optimization. The time required to detect movement (or
lack of movement) between subnets, and to obtain (or continue to
use) a valid IPv4 address represents a significant fraction of
the overall latency resulting from movement between points of
attachment on the network. As a result, optimization of
detection of network attachment in IPv4 hosts is helpful, to the
extent that it is achievable.
In order to achieve these goals, this document specifies
procedures conducted on connection to a network. On
disconnection from a network, there is no need to take action
until the host is reconnected, since it is typically not
possible for a host to communicate until it has obtained
connectivity. Therefore, contrary to [RFC2131] Section 3.7, no
action need be taken on network disconnection.
1.1. Requirements
In this document, several words are used to signify the requirements of
the specification. These words are often capitalized. The key words
"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in [RFC2119].
1.2. Terminology
This document uses the following terms:
DHCP client
A DHCP client or "client" is an Internet host using DHCP to
obtain configuration parameters such as a network address.
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DHCP server
A DHCP server or "server" is an Internet host that returns
configuration parameters to DHCP clients.
Routable address
In this specification, the term "routable address" refers to any
address other than an IPv4 Link-Local address. This includes
private addresses as specified in [RFC1918].
2. Framework
This document specifies a procedure to be performed for IPv4 network
attachment detection that depends on three phases: determination of the
"most likely" point of attachment, a reachability test phase, and an
IPv4 address acquisition phase.
The following basic principles are suggested:
[1] Utilization of link layer hints. Link layers such as IEEE 802
[IEEE802] provide hints about whether a host remains on the same
subnet despite changing its point of attachment, or even whether
the host is connected to an adhoc or infrastructure network.
Where available, these hints can be used to guide host behavior
- with the understanding that they are not infallible and
therefore that the host should be capable of making the correct
determination even in the presence of misleading hints. Link
layer hints are described in more detail in Appendix A.
[2] Link-Local IPv4 addressing as a mechanism of last resort. In
existing implementations of [IPv4LL], once a Link-Local IPv4
address is assigned, the DHCPv4 server may not be queried again
for 5 minutes. As a result, the inappropriate assignment of a
Link-Local IPv4 address results in an extended period of limited
connectivity. For a host that may change its point of
attachment more frequently than every 5 minutes, the
inappropriate assignment of an Link-Local IPv4 address is more
than just an annoyance - it can result in an ongoing inability
to connect. As a result, this document suggests that hosts
behave conservatively with respect to assignment of Link-Local
IPv4 addresses, using them only as a last resort.
2.1. Most Likely Point of Attachment
On connecting to a new point of attachment, the host attempts to
determine the "most likely" configuration associated with the new point
of attachment.
In order to determine the "most likely" point of attachment it is
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assumed that the host is capable of obtaining and writing to stable
storage parameters relating to networks that it connects to, including:
[1] IP and link layer hints associated with each network. For
details, see Appendix A.
[2] The IP and MAC address of default gateway(s) on each network.
By matching the received hints against information previously
collected, the host may be able to make an educated guess of
which network it has attached to. Where no additional
information is available, by default the host assumes that the
"most likely" point of attachment is the network to which it was
most recently connected.
If the host has a valid routable IPv4 address on the "most
likely" point of attachment, the host performs a reachability
test as described below. If the reachability test is not
successful, or if the host does not have a valid routable IPv4
address on the "most likely" point of attachment, the host
proceeds to the IPv4 address acquisition phase.
2.2. Reachability Test
The purpose of the reachability test is for the host to quickly
determine whether it is connected to a network on which it had
previously obtained a still valid routable IPv4 address. The test is
performed by attempting to verify reachability of a previously
configured primary default gateway on a former point of attachment. If
the test is successful, the host may continue to use a valid routable
IPv4 address without having to re-acquire it.
The host skips the reachability test and proceeds to the address
acquisition phase in the following circumstances:
[a] If the host does not have information on the default gateway on
the network.
[b] If the host does not have a valid routable IPv4 address on the
network. Since Link-Local IPv4 addresses are a last resort,
these addresses do not count as a valid routable IPv4 address.
2.2.1. Packet Format
To perform the reachability test, an ARP Request SHOULD be sent, using
the host's MAC address as the source, and the broadcast MAC address as
the destination. The host sets the target protocol address (ar$tpa) to
the IPv4 address of the primary default gateway, and uses its own MAC
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address in the sender hardware address field (ar$sha). Since the host
has not yet confirmed the subnet on which it is attached, it MUST set
the sender protocol address field (ar$spa) to 0.0.0.0. This prevents
poisoning of the ARP cache with a (potentially invalid) sender IPv4
address.
If a valid ARP Response is received, the MAC address in the target
hardware address field (ar$tha) and the IPv4 address in the target
protocol address field (ar$tpa) are matched against the list of networks
and associated default gateway parameters. If a match is found, then
if the host has a valid IPv4 address lease on the matched network, the
host continues to use that IPv4 address, subject to the lease re-
acquisition and expiration behavior described in [RFC2131], Section 4.4.5.
Checking for a match on both the IPv4 and MAC addresses of the default
gateway allows the host to confirm reachability even where the host
moves between two private networks. In this case the IPv4 address of
the default gateway could remain the same, while the MAC address would
change, so that both addresses need to be checked.
Sending an ICMP Echo Request to the default gateway IPv4 address does
not provide the same level of assurance since this requires an ARP
Request/Response to be sent first, and typically does not allow the MAC
address to be checked as well. It therefore SHOULD NOT be used as a
substitute. Where a host moves from one private network to another, an
ICMP Echo Request can result in an ICMP Echo Response even when the
default gateway has changed, as long as the IPv4 address remains the
same. This can occur, for example, where a host moves from one home
network using prefix 192.168/16 to another one. In addition, if the
ping is sent with TTL > 1, then an ICMP Echo Response can be received
from an off-link gateway.
If the initial ARP Request does not elicit a Response, the host waits
200ms and then sends another ARP Request. If no ARP Response is
received in response to this second Request, the host proceeds to the
IPv4 address acquisition phase. If a valid ARP Response is received,
but cannot be matched against known networks, the host assumes it has
moved subnets and moves on to the address acquisition phase.
2.3. IPv4 Address Acquisition
If the host has a valid cached configuration on the "most likely" point
of attachment, but is unable to confirm reachability to the primary
default gateway, then the host seeks to verify the cached configuration
by entering the INIT-REBOOT state, and sending a DHCPREQUEST to the
broadcast address as specified in [RFC2131] Section 4.4.2.
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If the host does not have a valid cached configuration, or had not
previously obtained a routable IPv4 address on the "most likely" point
of attachment, then the host enters the INIT state and sends a
DHCPDISCOVER packet to the broadcast address, as described in [RFC2131]
Section 4.4.1. If the host does not receive a response to a DHCPREQUEST
or DHCPDISCOVER, then it retransmits as specified in [RFC2131] Section 4.1.
As discussed in [RFC2131], Section 4.4.4, a host in INIT or REBOOTING
state that knows the address of a DHCP server may use that address in
the DHCPDISCOVER or DHCPREQUEST rather than the IP broadcast address.
However, sending a DHCPREQUEST to the unicast address when in INIT-
REBOOT state is not appropriate since it is possible that the client has
moved to another subnet, and therefore the DHCPREQUEST needs to be
forwarded to and from the DHCP server by a DHCP Relay so that the
response can be broadcast. This ensures that the host will receive a
response regardless of whether the cached IP address is correct for the
network to which it has connected.
As described in [IPv4LL] Section 1.7, use of a routable address is
preferred to use of a Link-Local IPv4 address. [RFC2131] Section 3.2
states that if the host possesses a valid routable IPv4 address on the
"most likely" network and does not receive a response after employing
the retransmission algorithm, the client MAY choose to use the
previously allocated network address and configuration parameters for
the remainder of the unexpired lease. This is preferable to assigning a
Link-Local IPv4 address if the host has good reason to believe that it
remains connected to a network on which it possesses a valid IPv4
address lease. This would be the case, for example, where a host has
received "hints" that it believes to be "strong". See Appendix A for
details.
If the host does not have a valid IPv4 address lease on the "most
likely" network and does not receive a response after employing the
retransmission algorithm, it MAY assign a Link-Local IPv4 address.
Since a Link-Local IPv4 address is often configured because a DHCP
server failed to respond to an initial query or is inoperative for some
time, it is desirable to abandon the Link-Local IPv4 address assignment
as soon as a valid IPv4 address lease can be obtained.
Where a Link-Local IPv4 address is assigned, experience has shown that
five minutes (see [IPv4LL] Appendix A.2) was too long an interval to
wait and try to obtain a routable IPv4 address via DHCP. A host which
has been configured with a Link-Local IPv4 address SHOULD periodically
attempt to obtain a routable IPv4 address via DHCP. The recommended
policy is to attempt to obtain an address via DHCP after waiting for
RECONF_INTERVAL, plus a random number of seconds, uniformly distributed,
between zero to RECONF_JITTER seconds.
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INTERNET-DRAFT DNAv4 11 September 20033. Constants
RECONF_INTERVAL 30 seconds
RECONF_JITTER 10 seconds
4. IANA Considerations
This specification does not request the creation of any new parameter
registries, not does it require any other IANA assignments
5. Security Considerations
Detection of Network Attachment (DNA) is typically insecure, so that it
is inadvisable for a host to adjust its security based on which network
it believes it is attached to. For example, it would be inappropriate
for a host to disable its personal firewall based on the believe that it
had connected to a home network.
ARP [RFC826] traffic is inherently insecure, so that the reachability
test described in Section 1.3 can be easily spoofed by an attacker,
leading a host to conclude that it remained attached to a former
network. Similarly, where DHCP [RFC2131] traffic is not secured, an
attacker could masquerade as a DHCP server, in order to convince the
host that it was attached to a particular network.
Where secure detection of network attachment is required, a host MAY
wish to skip the ARP-based reachability test entirely since it cannot be
secured, and go immediately to the IPv4 address acquisition phase,
utilizing authenticated DHCP [RFC3118].
6. References6.1. Normative References
[RFC791] Postel, J., "Internet Protocol", RFC 791, USC/Information
Sciences Institute, September 1981.
[RFC792] Postel, J., "Internet Control Message Protocol", RFC 792,
USC/Information Sciences Institute, September 1981.
[RFC826] D. Plummer, "An Ethernet Address Resolution Protocol -or-
Converting Network Addresses to 48-bit Ethernet Address
for Transmission on Ethernet Hardware", STD 37, RFC 826,
November 1982.
[RFC1256] Deering, S., "ICMP Router Discovery Messages", RFC 1256,
Xerox PARC, September 1991.
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Appendix A - Hints
In order to assist in IPv4 network attachment detection, information
associated with each network may be retained by the host. Based on IP
and link-layer information, the host may be able to make an educated
guess as to whether it has moved between subnets, or remained on the
same subnet. If it is likely to have moved between subnets, the host
may have an educated guess as to which subnet it has moved to. The term
"strong hint" refers to information which provides an unambiguous
indication of the network to which a host has connected. "Weak hints"
involve information which is inconclusive.
IPv4 ICMP Router Discovery messages [RFC1256] provide information
directly relevant to determining the network to which a host has
connected. As such, information gleaned from Router Advertisements can
be considered a "strong" hint.
For networks running over PPP [RFC1661], "weak" hints include the link
characteristics negotiated in LCP, and the associated phone number. The
IP parameters negotiated in IPCP are considered a "strong" hint.
On IEEE 802 wired networks, hints include link-layer discovery traffic
as well as information exchanged as part of IEEE 802.1X authentication.
Link-layer discovery traffic includes Link Layer Discovery Protocol
[LLDP] traffic as well as network identification information passed in
the EAP-Request/Identity or within an EAP method exchange.
For example, LLDP advertisements can provide information on the IP
address or VLANs supported by the device. These hints, if provided, are
considered "strong"; all other hints are considered "weak". When used
with IEEE 802.1X authentication, the EAP-Request/Identity exchange may
contain the name of the authenticator, also providing information on the
potential network. Similarly, during the EAP method exchange the
authenticator may supply information that may be helpful in identifying
the network to which the device is attached.
In IEEE 802.11 [IEEE80211] stations provide information in Beacon and/or
Probe Response messages, such as the SSID, BSSID, and capabilities, as
well as information on whether the station is operating in
Infrastructure or Adhoc mode. As described in [Congdon], it is possible
to assign a Station to a VLAN dynamically, based on the results of IEEE
802.1X [IEEE8021X] authentication. This implies that a single SSID may
offer access to multiple VLANs, and in practice most large WLAN
deployments offer access to multiple subnets.
Thus, associating to the same SSID is a necessary, but not necessarily a
sufficient condition, for remaining within the same subnet. While a
Station associating to the same SSID may not necessarily remain within
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the same subnet; on the other hand, a Station associating to a
different SSID is likely to have changed subnets. In order to provide
additional guidance on the subnets to which a given AP offers access,
additional subnet-related Information Elements (IEs) have been proposed
for addition to the IEEE 802.11 Beacon and Probe Response messages.
Such hints are considered "strong"; all other IEEE 802.11 hints are
considered "weak".
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Copyright (C) The Internet Society (2003). All Rights Reserved.
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herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
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Open issues
Open issues relating to this specification are tracked on the following
web site:
http://www.drizzle.com/~aboba/DNA/dnaissues.html
Expiration Date
This memo is filed as <draft-ietf-dhc-dna-ipv4-01.txt>, and expires
February 22, 2004.
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